1. Field of the Invention
The present invention relates to accumulators having a shell and a bladder dividing the interior of the shell into a liquid chamber and a gas chamber, the peripheral edge of the bladder being attached to the shell.
2. Description of the Prior Art
Such an accumulator has been conventionally known, for example, from Japanese Laid-Open Utility Model Application No. 168001/89. In this accumulator, the radii of curvature of an inner wall of the shell facing toward the gas chamber and an inner wall facing toward the liquid chamber are substantially the same, and thus the shell as a whole is spherical.
In such an accumulator, the volume of the gas chamber and the volume of the liquid chamber have a close relationship with the size (diameter) of the bladder, and since the diameter of the bladder is determined by the volumes of the gas chamber and the liquid chamber, the diameter is difficult to increase. Therefore, in such an accumulator, since the diameter of the bladder relative to the internal volume of the shell is relatively small and the deflection of the bladder with a variation in pressure in the gas chamber, i.e., a variation in volume, is relatively large, and since the radius of curvature of the curved portion of the peripheral edge of the bladder is also small, a problem of durability for the bladder results.
When the bladder is formed by a lamination of a bladder body made of a rubber and a gas impervious member formed from a material different in elongation from the bladder body such as in the accumulator disclosed in Japanese Laid-Open Utility Model Application No. 168001/89, the durability of the bladder is largely governed by the amount of deflection and hence, the above durability problem is significant.
In accumulators used in anti-lock brake control devices for automobiles, hydraulic boosting devices for brake devices, and traction control devices for an automobile by use of a brake, the bladder divides the interior of the shell into a liquid chamber for accumulating a working liquid such as a brake fluid and a gas chamber. In order to prevent gas from permeating into the liquid chamber from the gas chamber, the bladder includes a member formed from a laminated fabric material which is affixed to at least one surface of a thin sheet element of metal or resinous material having a small gas-permeability, the member being embedded in a bladder body made of an elastomer, or alternatively, the member is affixed to one surface of the bladder body. With such a construction for the bladder, the permeation of gas is inhibited by the thin sheet element of the metal or resinous material, and the strength of the thin sheet element is reinforced by the laminated fabric material.
However, a thin sheet element made of metal has extremely low values of elongation and flexibility and hence, fatigue induced due to the repeated displacement of the bladder in response to an increase and decrease in brake fluid pressure in the liquid chamber is quite likely. The spread of fatigue type cracks can be inhibited by the laminated fabric material, but it is difficult to positively prevent gas permeation through the bladder.
In addition, in the above-mentioned prior art, the resinous material of the thin sheet element is polyvinylidene fluoride or chloride and an element of such a resinous material has low elongation at a low temperatures. Hence, it is difficult to positively prevent the generation of cracks due to repeated displacement of the bladder. The polyvinylidene fluoride or chloride has a high resistance to solvents and therefore it is difficult to improve the cold temperature resistance of the material by the addition of a plasticizer.
Moreover, the elongation of the laminated fabric material is extremely small and hence, the displacement of the bladder is restrained by the laminated fabric material. As a result, the accumulating capacity of the accumulator is necessarily reduced.
It has been found that to solve such a problem, a material exhibiting a large elongation at low temperatures may be used to inhibit the gas permeation. For example, if the gas-impervious member is formed from a polyvinyl alcohol which contains glycerine, the above requirement can be met. However, when the subject working liquid contains an ethylene glycol alkyl ether or the like such as a brake fluid and if a gas-impervious member is formed using only the polyvinyl alcohol which contains glycerine as described above, the gas impervious member becomes hardened and tends to crack particularly at low temperatures since the immersion of the member in the working liquid results in the extraction of the glycerine from the member.
Further, in the above-mentioned prior art, the gas-impervious member is formed by affixing a second membrane of laminated fabric material to at least one surface of a first membrane composed of a single layer of a synthetic resin, the first membrane having a greater elongation than the second membrane. Therefore, since the repeated displacement of the bladder causes relatively large shearing forces to act on the adhered surfaces of the first and second membranes, delamination of the adhered surfaces occurs resulting in an increased gas-permeability.